In the World of the High Tech Redneck, the Graybeard is the old guy who earned his gray by making all the mistakes, and tries to keep the young 'uns from repeating them. Silicon Graybeard is my term for an old hardware engineer; a circuit designer. The focus of this blog is on doing things, from radio to home machine shops and making all kinds of things, along with comments from a retired radio engineer running from tech or science news to economics; from firearms to the world at large.

Friday, August 8, 2014

A Little More About .308 vs 7.62x51

Thanks to comments from McThag and a post by American Mercenary for reminding me of the differences in how the pressures are measured between the two cartridges: .308 in PSI and 7.62x51 in CUP (Copper Units of Pressure, an older way of specifying chamber pressures). I've looked into this subject before and came to the conclusion the BulkAmmo poster mentioned

My story is probably newer than most everyone's since I'm still a relatively new shooter, getting more involved in the sport in '09 than earlier in my life. The story starts when I bought a DPMS LR-308 in 2010. To shorten the story, the first box of .308 ammo I got from a gun show seller was marked both .308 Win and 7.62x51. Even then I knew they weren't exactly the same so I bought some commercial .308 ammo and spent some time trying to figure out if I could use the NATO spec ammo in my gun. DPMS marks the gun as .308 Win, and warns against using anything else. Quoth DPMS:

While you can physically fire either 308 or 7.62 NATO from a 308, you will see reduced accuracy from increased wear in the throat of the rifling and a higher chance of erratic cycling, including a higher chance of failures to extract. It is for this reason we recommend using only commercial 308 in a 308 barrel.

DPMS does not recommend or warranty the use of 7.62x51 ammo in a .308 chamber.

Pretty serious, huh? If you use Mil ammo in your DPMS .308 rifle, you void the warranty. After cruising forums and various experts, I saved this bit of wisdom.

OK, lets clear up this .308/7.62 business once and for all. Both rounds can be used in either chamber, however there are a few things you need to be aware of if using .308 in a 7.62 chamber.

First, realize that .308 and 7.62 have identical external dimensions. Because of this they can both be used interchangeably.

That said, it's important to note that a .308 case has a larger internal dimension than a 7.62 case. The walls on a 7.62 case are thicker than that on the .308. The result is that the 7.62 case is stronger, but the .308 case can have a higher potential load.

This is important to note because for the .308 the combination of a thinner wall, with a higher charge, means the case can be more easily deformed when fired. This is why the .308 chamber is a tighter spec, in order to restrict expansion of the case.

The 7.62 chamber is a slightly higher tolerance (which can aid in reliability, as it is not incredibly picky about round dimensions) which is fine for the thicker walled 7.62 case (thicker case = less likely to deform), but may present an issue when using .308 with a high charge (easier to deform).

Also of note is that while .308 is CAPABLE of a higher charge, it is often not loaded to max spec by manufacturers. 7.62 on the other hand often is loaded closer to it's max spec, and as such they tend to be "hotter" than commercial .308 rounds.

You can use 7.62 rounds in a .308 chamber with no problem (cheap surp might be a little tight though), and you can use .308 rounds in a 7.62 chamber as long as you don't reload using too high a charge. If you DO use an overly high charge for a .308 round in a 7.62 chamber, you are likely to deform the case, or tear the head off the case.

If you don't reload at all, you don't need to worry about it (unless you bought handloads).

So sure, ideally it would be good to use the round your chamber was made for, but by no means do you have too. For most, they can use either chamber, either round, and will never have any issue.

And remember people, this is the internet. Do not simply believe everything you read just because it may sound good... even from me (and even though the above is accurate). Please go out and research for yourself in order to confirm what I'm telling you.

Although I've had that in text file on my computer for a few years, I didn't keep track of where it came from. If it's yours, I'd be happy to credit it to you. For everyone else, heed the last paragraph.

You touched on it, but didn't continue expressing the thought: Since 7.62X51 brass is thicker, especially at the case head, internal case volume is significantly lower in 7.62X51 brass than .308 Winchester brass. (The reason for this is the violent extraction particular to full auto firearms, especially the 7.62 miniguns, which extract while the firing chamber still contains pressure from firing; a minigun will rip the case heads off .308 brass.) And, by "significantly" I mean "enough to cause a pressure difference."

This reduced case volume leads to higher pressures if using the same powder charge in 7.62X51 as one uses in .308 Winchester cases when one reloads. Most reloading manuals mention this (all of my manuals are older, so I don't know what's in the newest editions), but I'm not aware if they stress it.

The point being, since greatest accuracy in almost any cartridge is achieved with the case about 95% full of powder (some powders, and WW296, a shotshell powder also used for magnum pistols is one example) come with a caution to load to at least 90% of case capacity in magnum pistol calibers to ensure proper powder powder ignition and burn) some reloaders may jump to the right side of the powder charge table without starting with lower charges and working up.

The reloading rule of thumb is start no higher than 85-90% of maximum listed charge and work up gradually, looking for pressure signs (which, SiG, might be a good topic for a future post). And, when working with .308 Win / 7.62X51 rifles, unless charges are well below maximum, it's a really good idea to keep your brass segregated. Not only will that help prevent possible pressure problems, the pressure differential from the same load in the different internally-sized cases will cause a velocity difference which affects accuracy; if you're seeing vertical stringing on the target, mixing 7.62X51 brass with .308 Win and reloading them to the same charge level can be one cause.

Holy Shit the ignorance of this author is going to get people hurt. The SAMMI specs list the 308 round as higher pressure than the NATO 7.62.51 round. This is inverese to the 5.56x45 versus the .223 whereas the 5.56x45 is higher pressure. The SAMMI/CIP maximum pressure for the .308 Win cartridge is 62,000 psi, while the 7.62x51 max is 50,000 psi Sheesh.

came across some 20/box ammo labeled "308 Win. 7.62x51" Fed. Lake City . What can this be safely be fired in - no reloading of brass? Never seen ammo labeled as such. I have two AR format rifles, one stamped "308" and the other barrel stamped "7.62 x 51 NATO".

I feel really uncomfortable with answering this because I have no idea what you have. What does the head stamp on the brass say?

What I get as the takeaway from this is that with a rifle marked ".308 Win", you can run NATO or .308 Win commercial and it'll be OK. If it says "7.62x51", you need to stick with that. That's the opposite of .223/5.56 where the NATO chambering will handle both and the .223 Remington chamber is only for that.

I have a DPMS marked ".308 Win" and have run both types in it, with no jams or no failures to cycle. (Well, I had one bad lot of Magtech, but they knew about it and replaced it with no questions asked).

Disclaimer: I'm just some dude on the Internet with a blog. I really hope this works, but there's no way I know what you've got.

Springfield armory states strongly that only 7.62 be used in the M1A. That according to the information sent with the rifle. They worn of thin primers used in the 308 rds that could result in "slam fires" caused by the firing pin strike at the load/locking stage of the cycle..

Use the M14 years ago but used military ammo. Any one have any bad moments using over the counter 308 in any of the M1 based rifles.

AS of today, 9/5/16...Springfield's web site says M1A can use 308 ammo. I'm pretty sure when they were building M1A'a with surplus M14 receivers and chambers they recommended 7.62x51, but not any more. I've shot both in my M1A with zero problems for the past year.

Having recently bought my second M1A, I called Springfield Armory to check on this (about a month ago). I was told specifically that they could use either .308 or 7.62x51. I repeated the question to verify, and was again told that I could use .308. There was nothing in the literature accompanying the rifle to indicate otherwise, but I was just checking to be sure.

I load 2 gr under max in both cases, 308 and nato. fired from the same rifle, and in every type of powder and bullet, Never had a problem, EVER then bump up to just under max, still no problem., Nato, VS 308, they all shoot the same and do the job . Just size the brass, and trim it , then load and shoot , 25 years of doing this, well it tell me something

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Retired radio engineer, follower of Christ, RF designer, mentor. Radio ham, home shop machinist, lapidary, silversmith, roadie cyclist, learning to be a rifleman, and home defender, - a guy with too many interests to keep track of.

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